Wireless communications devices, for instance, mobile phones have become popular remarkably in recent years with their functions and services improved increasingly. Taking a mobile phone as an example, there are various systems for mobile phones, for instance, EGSM (Extended Global System for Mobile Communications) and DCS1800 (Digital Cellular System 1800) systems widely used mostly in Europe, a PCS (Personal Communications Services) system used in the U.S., and a PDC (Personal Digital Cellular) system used in Japan. In such mobile phones of digital communications systems, high-frequency switches are used to switch the connection between a transmitting circuit and an antenna and the connection between a receiving circuit and an antenna.
One example of the high-frequency switches is disclosed by Japanese Patent Laid-Open No. 2-108301. This high-frequency switch comprises a diode placed between a transmitting circuit and an antenna, and a λ/4 phase shift line placed between an antenna and a receiving circuit, the receiving circuit side of the λ/4 phase shift line being grounded via a diode, thereby constituting a λ/4-type switch circuit for switching signal paths by a bias current flowing through each diode.
According to recent rapid expansion of mobile phones, however, a frequency band allocated to each system cannot allow all users to use their mobile phones in major cities in advanced countries, resulting in difficulty in connection and thus causing such a problem that mobile phones are sometimes disconnected during communication. Thus, proposal was made to permit users to utilize a plurality of systems, thereby increasing substantially usable frequencies, and further to expand serviceable territories and to effectively use communications infrastructure of each system.
In such circumstances, dual-band mobile phones, triple-band mobile phones, etc. have been proposed as mobile phones having new systems. While a usual mobile phone comprises only one transmitting/receiving system, the dual-band mobile phone comprises two transmitting/receiving systems, and the triple-band mobile phone comprises three transmitting/receiving systems. With these structures, users can choose and utilize available transmitting/receiving systems among a plurality of systems. In the dual-band mobile phone and the triple-band mobile phone, there is a high-frequency switch for switching the connection between an antenna and a transmitting circuit or a receiving circuit by time division, so that bi-directional communications can be carried out with one antenna shared in transmitting/receiving systems.
The inventors already proposed in Japanese Patent Laid-Open Nos. 11-225089 and 11-313003 a high-frequency switch module comprising a combination of a branching circuit (diplexer) for separating high-frequency signals of a plurality of frequency bands and a high-frequency switch, with a function of switching the transmitting circuit and the receiving circuit of a plurality of communications systems as a high-frequency switch capable of utilizing a plurality of communications systems, the high-frequency switch module being constituted by a multi-layered module integrated with the branching circuit (diplexer) the high-frequency switch circuit, etc.
FIG. 21 shows an equivalent circuit of a high-frequency switch disclosed by Japanese Patent Laid-Open No. 2-108301. To connect an antenna terminal ANT to a transmitting circuit TX, positive voltage is given from a power supply means (control circuit) to a terminal VC1. With a DC signal cut by capacitors 70, 71, 73, 74 and 79, positive voltage given by the control circuit is applied to a circuit comprising diodes 77, 78 to turn the diodes 77, 78 to an ON state. With the diode 77 in an ON state, there is low impedance between the transmitting circuit TX and the connection point IP1. Also, with a diode 78 in an ON state, the distributed constant line 75 is grounded at high frequencies, causing resonance, resulting in the extremely high impedance of the receiving circuit RX viewed from the side of the connection point IP1. As a result, the transmitting signal from the transmitting circuit TX is sent to the antenna terminal ANT without leaking to the receiving circuit RX.
However, because the diode 77 placed in series between the antenna terminal ANT and the transmitting circuit TX acts as a resistor in an ON state, the transmitting signal suffers from large loss. Because the diode through which a bias current should flow at transmitting consumes electricity from the battery, the mobile phone has a short period of communicatable time, resulting in difficulty in achieving low electricity consumption. Further, because parts such as diodes and capacitors for cutting a DC signal are needed, high-frequency switch modules are constituted by inevitably large multi-layered modules, making it difficult to provide small and lightweight wireless communications devices of dual band or more.